Rust Ffi Cross Language

Use when a Rust crate needs to be called from Python, Node.js, Swift, or Kotlin without a separate FFI codegen step.

Rust FFI Binding Generation

Overview

cbindgen reads Rust extern "C" blocks and emits idiomatic C headers. Pair with language-specific binders: cffi (Python), node-ffi-napi (Node), a .swift wrapper (Swift), or JNA (Kotlin/Java).

Procedure

1. cargo add --build cbindgen and add a build.rs that calls cbindgen::generate().write_to_file("mycrate.h").
2. Annotate every #[no_mangle] extern "C" function with #[repr(C)] and use only POD-compatible types (ints, pointers, slices as *const/*mut).
3. For strings, pass *const c_char (null-terminated) or (*const u8, usize) byte-slice pairs to avoid allocator coupling.
4. Build: cargo build --release. Distribute the .so/.dylib/.dll alongside the generated header.
5. Language binders:
   • Python: ffi.cdef(open("mycrate.h").read()); lib = ffi.dlopen("libmycrate.so")
   • Node: define via ffi-napi using the same C signatures.
   • Swift: import Glibc / Darwin and call through the .so.
6. Never cross an allocator boundary — Rust must allocate+deallocate (use a free function exported the same way), or copy into a caller-owned buffer.

Pitfalls

• Panics across the FFI boundary are UB; wrap entry points in std::panic::catch_unwinding.
• String/Vec are NOT C-compatible — leak or segfault.
• Debug vs release ABIs differ; always ship release builds.

Verification

• cbindgen --config cbindgen.toml --crate mycrate --output mycrate.h exits 0.
• strings libmycrate.so | grep <exported_symbol> shows the symbol.
• Python: python3 -c "import mymod; print(mymod.add(2,3))" prints 5.